As industrial machinery increases in power and decreases size with the advance of industrial technology, the conditions under which a lubricating oil is used have become increasingly severe. Furthermore, in order to reduce the burden on the environment, the policy of saving energy and environmental resources is also applicable to lubricating oils. With such social trends, even better performance, such as improved thermal stability, excellent oxidation stability, higher viscosity index, lower volatility and better fluidity at low temperatures, have been required of lubricating oils, and mineral oil-based lubricating oils have gradually been replaced with synthetic lubricating oils. It is noted that esters, in particular hindered esters, are used for engine base oils, grease base oils, working fluids, turbine oils, heat carrier oils, metal working oils and refrigerating oils.
Since the above-mentioned various lubricating oils are used for the purposes or under conditions which keep them almost always in contact with a metal, various requirements for oil performance should always be met in the presence of a metal.
Esters, however, are subject to thermal decomposition in the presence of a metal at high temperatures, causing a problem of generating carboxylic acids or metal carboxylates. In particular, the above problem occurs with a lubricating oil used for compressors for refrigeration machines, such as refrigerators, where a hindered ester is used as a refrigerating oil for a substitute refrigerant.
Also, the use of chlorodifluoromethane (HCFC22), which is now used for room air conditioners, packaged air conditioners and refrigeration machines for industrial use, has recently been decided to be restricted or banned in the near future, and a hydrofluorocarbon mixture has been proposed as a candidate for an alternative. The use of such hydrofluorocarbon mixture increases the pressure inside a compressor when compared with the use of 1,1,1,2-tetrafluoroethane (HFC134a) which is currently used for refrigerators. In particular, when it is used for a rotary compressor or a scroll compressor, the refrigerating oil used therefor is exposed to severe thermal conditions. Thus, the concern about the above problem is further increased.
In order to avoid these problems, the use of lubricating oils which are thermally stable in the presence of a metal, such as alkylated diphenylethers and perfluoropolyethers, has been considered. However, these compounds are generally expensive and when alkylated diphenylether is used as a refrigerating oil, its insolubility in hydrofluorocarbon refrigerants becomes a problem.
For the above reasons, a lubricating oil which is thermally stable in the presence of a metal and less expensive has been in demand.
Incidentally, the fact that a metal (iron) exerts an influence on the thermal stability of an ester compound is described in R. L. COTTINGTON et al. [ASLE Trans. 12, 280-286 (1969)] and SATISH K. NAIDU et al. [Wear, 121(1988)211-222].
It has been pointed out that the addition of TCP (tricresylphosphate) to the base oil is effective to improve thermal stability in the presence of a metal (Report by the above mentioned R. L. COTTINGTON et al.). However, there is no teaching at all about the relation between the thermal stability in the presence of a metal and the structure of ester compounds. That is, neither the feasibility for improving the thermal stability of an ester compound used as a base oil nor the guideline for the selection of ester structures, if such improvement has been possible, is taught.
On the other hand, it has been reported in various publications that ester compounds having a particular structure, that is, those formed from branched carboxylic acids, have an excellent performance as a lubricating oil.
Specifically, for obtaining ester compounds having excellent oxidation stability, use of fatty acids having 2 side chains at the .alpha.- or .beta.- position to the carbonyl group, as the starting material for hindered esters, is disclosed in the specifications of U.S. Pat. No. 3,115,519, U.S. Pat. No. 3,282,971, British Patent No. 999099, and British Patent No. 1028402. Also, Japanese Patent Laid-Open No. 55-105644 and some other publications disclose that hindered esters prepared from 3,5,5-trimethylhexanoic acid and a linear carboxylic acid at a ratio of 90:10 to 10:90 is excellent in oxidation stability. Also, Japanese Patent Laid-Open No. 5-17787 and some other publications disclose that hindered esters prepared from 5-90% neopentyl branched fatty acids and 95-10% linear fatty acids and/or .alpha.-branched fatty acids are excellent in heat resistance. However, no discussion is made as to the heat resistance in the presence of a metal.
Moreover, in Japanese Patent Laid-Open No. 6-158079, a carboxylic acid ester having a branch at the 2- or 3-position is described as a lubricating oil composition having excellent oxidation stability and a good viscosity index, but the publication also mentions that a carboxylate having a branched structure and a linear carboxylate are comparable to each other and have no difference in oxidation stability.
As mentioned above, there have been no reports which discuss the thermal stability of an ester compound in the presence of a metal.
In the field of refrigerating oils, esters to be used in conjunction with hydrofluorocarbons containing HFC 32 are disclosed in Japanese Patent Laid-Open Nos. 5-17789, 5-32985, 5-239480, 6-17073, and 8-502769, and the ratio of branched acyl groups to the entire acyl groups is specified in some of the above publications. However, all these publications only mention the compatibility with hydrofluorocarbons containing HFC 32.
Also, esters for which the ratio of branched acyl groups to the entire acyl groups is specified are disclosed in Japanese Patent Laid-Open Nos. 3-200895, 4-311797, 4-314793, and 5-209171. However, these publications only mention the compatibility with HFC134a and the hydrolysis resistance.
In Japanese Patent Laid-Open Nos. 3-217493 and 5-25484, esters for which hydroxyl value is specified are disclosed. These publications only mention wear resistance and elution of PET oligomer.
Moreover, Japanese Patent Laid-Open No. 6-108076 discloses esters for which the ratio of branched acyl groups to the entire acyl groups and hydroxyl value are specified. However, it only mentions hydrolysis resistance in the presence of HFC134a.
As described above, there are no reports on the thermal stability of esters in the presence of hydrofluorocarbons containing HFC32.